Low noise image intensifier

ABSTRACT

An image intensifier of otherwise conventional construction is improved by an inwardly directed extension of the envelope, projecting from one edge of the focus electrode and extending toward the anode. The focus electrode is a conductive coating on the internal surface of the envelope and is, in effect, extended by a conductive coating applied to the inwardly facing surface of the envelope extension. That extension terminates in a metal ring. With the structure, the surface separation or leakage path between the metal ring termination of the focus electrode and the anode is substantially greater than that otherwise existing between those electrodes. The increased separation of these electrode elements along the surface of the envelope minimizes the tendency toward field emission and its contribution to background noise. A high resistance coating or an electron shield protects the shoulder of the envelope from electron bombardment and eliminates that source of background noise.

United States Patent [72] Inventor Constantin S. Szegho Chicago, Ill.[21 Appl. No. 756,443 [22] Filed Aug. 30, 1968 [45] Patented May 4, 1971[73] Assignee Zenith Radio Corporation [54] LOW NOISE IMAGE INTENSIFIER8 Claims, 4 Drawing Figs.

[52] US. Cl 313/101, 250/213, 313/65, 313/313 [51] Int. Cl ..H0lj 39/00,1-l0lj H52 [50] Field of Search 313/64, 65, 67, 74, 82, 94, 101, 102,58, 59, 204, 313; 250/213 [56] References Cited UNYTED STATES PATENTS3,026,437 3/1962 Niklas 313/65 3,225,204 12/1965 Schagen 313/653,280,356 10/ 1966 Stoudenheimer 313/65 Primary ExaminerJames W.Lawrence Assistant Examiner-David OReilly Attorney-Francis W. CrottyABSTRACT: An image intensifier of otherwise conventional construction isimproved by an inwardly directed extension of the envelope, projectingfrom one edge of the focus electrode and extending toward the anode. Thefocus electrode is a conductive coating on the internal surface of theenvelope and is, in effect, extended by a conductive coating applied tothe inwardly facing surface of the envelope extension. That extensionterminates in a metal ring. With the structure, the surface separationor leakage path between the metal ring termination of the focuselectrode and the anode is substantially greater than that otherwiseexisting between those electrodes. The increased separation of theseelectrode elements along the sur' face of the envelope minimizes thetendency toward field emission and its contribution to background noise.A high resistance coating or an electron shield protects the shoulder ofthe envelope from electron bombardment and eliminates that source ofbackground noise.

PATENTEU M 4m 3577.027

'00. HQ ||b Ob all Constantin S. Szegho Attorney LOW NOlSlE IMAGEHNTENSIFIER BACKGROUND OF THE INVENTION The present invention isdirected to image intensifiers and is most particularly concerned withimprovements in their noise properties. Obviously, any image converteror translating device benefits from improved noise properties so theinvention may be utilized with any type of image intensifier whether itbe arranged to respond to visible light, X-ray, infrared, gamma, neutronenergy or any other form of excitation.

The invention is particularly valuable in the field of nuclear medicalexamination in which radioisotopes energize an image converter theresponse of which is in the form of scintillations of an input layersuch as a cesium iodide crystal. To obtain a useful output it isnecessary that the scintillations be integrated and the need forintegration makes clear that a high noise ambient within the intensifierwill destroy its usefulness.

The causes of background noise in an image converter are manifold butprincipal offending phenomena are undesired emissions within the device.

In the usual converter the focus electrode which, in conjunction withthe anode, focuses and accelerates an electron image to impingeupon afluorescent output screen is a conductive layer on the internal surfaceof the envelope. This is a very thin layer and the edge which faces theanode assembly is generally jagged. The anode, which also includes aconductive layer on the inner surface of the envelope, is at a very highpotential relative to the focus electrode and consequently if theadjacent edge of the focus electrode is, in fact, jagged, conditionsexist that are favorable to field or cold emission. Where fieldemissionis experienced, the electrons released may find their way directly tothe output screen and contribute noise to the image or they may bombardthe glass envelope producing a fluorescence. The resulting light mayreflect onto the photoemissive cathode, giving rise to further electronemission which represents additional contributions to background noise.

One previous effort to improve matters in these regards is the subjectof U.S. Pat. No. 3,026,437, issued Mar. 20, 1962, in the name of W. F.Niklas and assigned to the assignee of the present invention. ltfeatures the application of a high resistance semiconductive coating,such as chrome oxide, to the internal portion of the envelope whichintervenes the focus electrode at one side and the wall coating of theanode structure on the other. The coating overlaps both of theseelectrode elements and establishes a uniform potential gradienttherebetween which tends to suppress cold emission. It has been found,however, that if the conductive coating is carelessly applied it maycrumble or flake and introduce undesired particulate matter within thetube envelope. lt is also found that if the resistance of the coating isnot uniform throughout, or if it should age nonuniforrnly in the coatedarea, the electron optics of the intensifier may become impaired. Thepresent invention is a distinct improvement, offering the possibility ofgreatly enhanced freedom from noise and freedom from unwantedparticulate foreign matter within the envelope.

Accordingly, it is a principal object of the invention to improve thebackground properties of an electron discharge device, specifically, animage converter.

It is a very particular object of the invention to provide an imageintensifier that has exceedingly low background noise and, therefore, isespecially suited for nuclear medical systems.

SUMMARY OF THE lNVENTlON An image intensifier typically comprises acylindrical envelope of insulating material having an electron-imagesource at one end, an image screen at the opposite end and a focuselectrode on the internal surface of the envelope intermediate suchends. An anode structure is located adjacent the image screen andincludes a conductive band on the envelope spaced from the focuselectrode. in accordance with the invention such an image converter isimproved by the provision of an inwardly directed extension of theenvelope projecting from the edge of the focus electrode that faces theimage screen in order to increase the surface separation or leakage pathof the focus electrode and the conductive band of theanode structure.

In one aspect of the invention, the envelope extension projects towardthe anode structure and terminates in a conductive ring. A conductivecoating on the inwardly facing surface of the envelope extension causesthis extension to be, in effect, a portion or continuation of the focuselectrode.

In another aspect of the invention, the shoulder portion of the envelopewhich usually leads from the location of the focus electrode to areduced diameter section which accommodates the image screen is coatedwith a thin film of calcium fluoride. This coating serves much the samepurpose as the chrome oxide coating of the prior art described above butis not subject to crumbling and avoids the possibility of contributingunwanted particulate matter within the tube envelope.

DESCRIPTION OF THE DRAWING The features of the present invention whichare believed to be novel are set forth with particularity in theappended claims. The invention, together with further objects andadvantages thereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawing,-in theseveral FIGS. of-which like reference numerals identify like elements,and in which:

FIG. 1 is a view in cross section of an image intensifier constructed inaccordance with the invention to exhibit improved noise properties; and

H65. 2, 3 and 4 are various modifications of the image intensifier.

DESCRlPTlON OF THE PREFERRED EMBODIMENTS The image converter of FIG. 1comprises an evacuated envelope formed of insulating material, usuallyglass, and cylindrical in shape having a uniform diameter throughoutmost of its length. The envelope may have any of a variety of shapesbut, as shown, it has two principal sections 10a and 10b which terminatein Kovar metallic flanges 11a and 11b which may be heliarc welded tojoin the envelope sections into an integrated enclosure. At one end, theenvelope has a'closure 10c which, of course, must be transparent to theradiation to which the image converter is to respond. Typically, it isformed of the same material as the remainder of the envelope. At theother end the envelope is closed by a reentrant press 10d which is areduced diameter section being connected with section 10b through ashoulder portion 10a.

An electron-image source is located at the large end of the envelope.This is a pickup screen 12 which approximates a sector of a sphere andis so oriented that its concave surface faces toward the reentrantportion 10d of the envelope. The pickup screen is a multilayer componenthaving a sphericallyshaped support member of aluminum or other materialthat is transparent to the radiation for which a response is to beobtained, here assumed to be X-ray simply for convenience ofillustration. Over the support is a layer which responds to thatradiation and emits light to activate a final layer which is aphotoemitter. These two layers are frequently separated by a barrierlayer for the purpose of preventing undesired chemical interactiontherebetween. The present invention does not concern the structure ofthepickup screen and suitable arrangements of this component are wellknown in the art. Suffice it to say that for an X-ray converter, theX-ray image sensitive layer is usually phosphor, such assilver-activated zinc sulfide, but for a nuclear converter the firstlayer may be formed of cesium iodide. The barrier layer, if employed,may be aluminum oxide and the photoemissive top layer is generally anantimony-cesium composition. The diameter of the pickup screen is aslarge as practical considerations permit.

At the opposite end of the envelope, there is an image screen whichusually is a fluorescent coating l3 applied to the glass closure plateof the reentrant envelope section d. The

pose of accelerating an electron image in the direction of screen 13 andfocusing the image to reach that screen by way of an aperture in theanode structure. This electron system accordingly includes a focuselectrode 114 in the form of a conductive metallic coating on theinternal surface of envelope section 10b. It is usually a film of copperor aluminum. One end of focus electrode M extends to metal flange lllband facilitates the application of a focus potential to that electrode.This is a-low potential of the order of a few hundred volts. The otherend of the focus electrode extends to an inwardly directed extension ofthe envelope to be discussed more particularly hereafter.

' The anode structure comprises a metallic electrode lb which has acylindrical portion lea having a spherical termination with a centrallydisposed aperture Mb. The cylindrical section leads to aconically-shaped section which terminates in a skirt portion lltic ofsuitable dimension to be accommodated by mounting over the reentrantreturn portion 10d of the envelope. The convex end face of anode cap 16afaces the concave surface of pickup screen 112 and the anode, asapparent in FIG. 11, encloses viewing screen l3 and is electricallyconnected to the conductive backing layer I30 thereof so that the screenis maintained at the same electrical potential as the anode. A portionof reentrant section llla' has a conductive band or coating led which isin circuit connection with anode 116 by means of contact springs orfingers the. This facilitates the application of anode, potential from alead shown schematically at l7 which penetrates the reduced diametersection of the envelope and connects with conductive band Md.

The structural details of the image converter as thus far described maybe entirely conventional and will be familiar to those skilled in theart. In operation, an image of X-radiation is focused on pickup screen112 where it is converted to an electron image that is acceleratedtoward anode to. Focus electrode M, in conjunction with anode l6,constitutes an immersion lens for the purpose of focusing an electronimage originating at pickup section 112 through anode aperture lob ontoviewing screen 113. While in prior structures the focus electrodecoating Ml extends far enough on the internal surface of envelope llllbto overlapanode l6, it will be apparent from FIG. I that the wallcoating 14 of the focus electrode need not extend that far with thestructure under consideration.

This difference from the usual prior art converter is involved in thestructural change introduced in the practice of preferred forms of thepresent invention for improving the noise background of the converter.

More specifically, the wall coating It extends to an inwardly directedextension 20 of envelope section 10b which projects in the direction ofanode 16. It may be likened to an inner wall of envelope section I012similar to and spaced inwardly of and concentric with the portion ofshoulder Illa that leads to the uniform diameter section 10b of theenvelope. Envelope extension 20 terminates in a conductive ring 21which, as illustrated, has an axially extending portion which terminatesin a flange disposed transversely of the tube axis. The surface ofenvelope extension 20 that faces inwardly is prov vided with aconductive coating 20a which overlaps or contacts wall coating Ml at oneend and conductive ring 211 at the other. This, in effect, causeselements 20 and 21 to constitute a continuation of focus electrode 141so that the composite electrode, as in the case of prior art structures,reaches beyond and is spaced from cap portion lea of the anode. It isdesirable that envelope extension 20 have a close spacing to thecircumscribing section of the converter envelope in order to havethe'least disturbance on the focusing field established betweenelectrodes 14 and 16. In one embodiment of the invention found to mostsignificantly improve the noise background of an image intensifier, thestructure had the following significant dimensions: 1 diameter ofenvelope section 10b 9-29/32 inches length of conductive coating onenvelope section 10b 6- 13/16 inches shoulder radius of envelope sectionlltle 2-% inches radius of envelope extension 20 inches axial length ofring 211 inches separation of ring 21 from envelope section We diameterof image screen 13 2 inches With this construction, the separation fromone edge of conductive band led of the anode structure along theinternal surface of the envelope to the adjacent edge of the focuselectrode, specifically to ring 21 of this electrode, is much greaterthan that found in prior structures. In the first place, wall coating 14is foreshortened with respect to the previous practices in the art andterminates prior to and, therefore, in nonoverlapping relation with thespherical end 16a of anode 16 whereas usually the wall coating extendsfarther in the direction of the anode to be in encompassing but spacedrelation thereto. Additionally, the uncoated and outwardly facingsurface of envelope extension 20 adds to the surface separation of theseelectrodes providing an enlarged gap having the dimension indicated byarrow S. The increased surface separation affords a major improvement insuppressing the tendency to field emission for a given potentialdifference between anode l6 and focus electrode 14.

Further and significant improvement in this regard results fromterminating the focus electrode in conductive ring 211 which, even inthe form represented in FIG. Limproves matters with respect to fieldemission when compared with the edge otherwise presented by wall coating14. Typically, such a conductive coating has a thickness of the order of10,000 A. and also it is generally ragged. If ring 21 is formed of metalhaving a thickness of one-sixteenth inch, and has a flange radius ofone-sixteenth inch, its edge dimension is large compared with thatotherwise presented by wall coatingl l and it may readily be processedto have a smooth surface with no raggedness. This, too, is a substantialimprovement in suppressing the tendency to field emission.

Still further improvement is obtained by the specifically differentconfiguration of ring 21, shown for. example in FIG. 2 where the ringhas a smoothly curved leading edge rather than the angular configurationof FIG. l.

While it is distinctly preferable, based on present experience, toterminate envelope extension 20 with conductive ring 2i, this may not benecessary and may be omitted in image intensifiers that do not have thesevere background noise requirement of converters constructed for use innuclear medical systems. By way of illustrating the difference in noiseproperties required in differing applications'of image converters, itmay be noted that a background level of 0.001 foot Lamberts isconsidered not only acceptable but quite good for X-ray imageintensifiers whereas an improvement of nearly I00 fold, which may beobtainedwith the structures of FIGS. 1 and 2 including conductive ring21, is required for nuclear intensifiers. For the X-ray and similarapplications where there is more tolerance for noise background, ring 21may be omitted and the focus electrode continued simply by the coating20a of envelope extension 20.

The discussion thus far has concerned itself principally with structuralimprovements which enhance the noise properties of the converter bysuppressing tendencies toward field emission. As explained above, acontribution to noise background may be made if electrons bombard theenvelope and produce fluorescence. This is most apt to occur byelectrons impinging on shoulder portion We of the envelope and may besuppressed or eliminated by the use of a wall coating 25 extending A;inches from conductive ring 16d of the anode structure in the directionof the focus electrode although it will not, as a practical matter,reach the focus electrode structure. Coating 25 may be of chrome oxideas disclosed in the above-identified Niklas patent in which case itwould be painted in place. Alternatively and preferably, in accordancewith the present invention, it is a thin evaporative film of calciumfluoride. This not only protects against fluorescence but is beneficialin that it is not susceptible to flaking or peeling. Because of itscontrollable conductivity, it may also facilitate the discharge ofsurface charges that could otherwise be present on shoulder section l0e.Of course, the calcium fluoride coating may be substituted directly forthe chrome oxide coating in image intensifier structures of the typeshown in the Niklas patent.

The modification of FIG. 3 features the use of an electron shield 26that is mechanically fastened to ring 21 by mounting screws 26a. Itextends toward anode l6 and is conical in shape, serving to protectenvelope section we from electron bombardment. Where such a shield isused, wall coating 25 may be omitted. Since the shield is removablyattached to ring 21, it is not installed until all other processingsteps for en velope section 10b have been completed. By installing theshield as the last step of the process, one has the advantage of greateraccess for inserting anode structure 16 into position.

As shown in FIG. 4, one may employ a conical conductive piece 28 as thecounterpart of the coating 20a on envelope extension 20 and shield 26 ofthe embodiment of F l0. 3. In such case, the enlarged end of member 28is slotted to form resilient fingers and its edge is configured to snapover and processing the section of the converter which includes imagescreen 13, the focus electrode and the anode structure. Of course, it isnecessary that the coating 14 on the internal surface of envelopesection 10b extend into the recess defined by the configuration toenvelope section 20 in order that insert 28 may be in circuit connectionwith wall coating 14.

While particular embodiments of the invention have been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects, and, therefore, the aim in the appended claims isto cover all such changes and modifications as fall within the truespirit and scope of the invention.

lclaim:

1. in an image converter comprising a cylindrical envelope of insulatingmaterial,

an electron-image source at one end,

an image screen at the opposite end,

a focus electrode on the internal surface of said envelope, and an anodestructure adjacent said screen including a conductive band spaced on theinternal surface of said envelope from said focus electrode, theimprovement which comprises an inwardly directed extension of saidenvelope projecting from the edge of said focus electrode facing saidscreen and between said fo'cus electrode and said opposite end, toincrease the effective separation of said focus electrode and said anodeband.

2. An image converter in accordance with claim 1 in which said envelopeextension projects from the edge of said focus electrode toward but inspaced relation to said anode;

and in which the inwardly facing surface of said envelope extension hasa conductive coating which connects with said focus electrode so thatsaid envelope extension serves as a continuation of said focuselectrode.

3. An image converter in accordance with claim 2 in which a conductivering, having an edge dimension large compared with that of said focuselectrode, is affixed to the free end of said envelope extension and isoverlapped by said coating on said envelope extension.

4. An image converter in accordance with claim 3 in which said envelopehas a reduced diameter section at said opposite end where said screenandsaid anode band are located and has a shoulder portion interconnectingsaid reduced diameter section to the portion of said envelope where saidfocus electrode is located, and in which said shoulder portion has acoating of high-resistance material to suppress undesired emissionattributable to electron bombardment of said shoulder portion.

5. An image converter in accordance with claim 4 in which said coatingon said shoulder portion is calcium fluoride.

6. An image converter in accordance with claim 3 in which said envelopehas a reduced diameter section at said opposite end where said screenand said anode band are located and has a shoulder portioninterconnecting said reduced diameter section to the portion of saidenvelope where said focus electrode is deposited, and in which afrustoconically-shaped conductive electron shield extends from said ringtoward said anode to shield said shoulder portion from electronbombardment.

7. An image converter in accordance with claim 1 in which said envelopehas a reduced diameter section at said opposite end where said screenand said anode band are located and has a shoulder portioninterconnecting said reduced diameter sectionto the portion of saidenvelope where said focus electrode is located, and in which afrustoconically-shaped conductive member is secured to said envelopeextension and extends toward said anode to constitute a continuation ofsaid focus electrode and an electron shield for shielding said shoulderportion from electron bombardment.

8. An image converter in accordance with claim 3 in which the edge ofsaid focus electrode closer to said anode structure is spaced from saidanode structure in the direction of said electron-image source; and inwhich said envelope extension and said conductive ring are dimensionedso that at least a portion of said ring is spaced from but is inoverlapping relation with respect to the end portion of said anodefacing said electron-image source.

2. An image converter in accordance with claim 1 in which said envelopeextension projects from the edge of said focus electrode toward but inspaced relation to said anode; and in which the inwardly facing surfaceof said envelope extension has a conductive coating which connects withsaid focus electrode so that said envelope extension serves as acontinuation of said focus electrode.
 3. An image converter inaccordance with claim 2 in which a conductive ring, having an edgedimension large compared with that of said focus electrode, is affixedto the free end of said envelope extension and is overlapped by saidcoating on said envelope extension.
 4. An image converter in accordancewith claim 3 in which said envelope has a reduced diameter section atsaid opposite end where said screen and said anode band are located andhas a shoulder portion interconnecting said reduced diameter section tothe portion of said envelope where said focus electrode is located, andin which said shoulder portion has a coating of high-resistance materialto suppress undesired emission attributable to electron bombardment ofsaid shoulder portion.
 5. An image converter in accordance with claim 4in which said coating on said shoulder portion is calcium fluoride. 6.An image converter in accordance with claim 3 in which said envelope hasa reduced diameter section at said opposite end where said screen andsaid anode band are located and has a shoulder portion interconnectingsaid reduced diameter section to the portion of said envelope where saidfocus electrode is deposited, and in which a frustoconically-shapedconductive electron shield extends from said ring toward said anode toshield said shoulder portion from electron bombardment.
 7. An imageconverter in accordance with claim 1 in which said envelope has areduced diameter section at said opposite end where said screen and saidanode band are located and has a shoulder portion interconnecting saidreduced diameter section to the portion of said envelope where saidfocus electrode is located, and in which a frustoconically-shapedconductive member is secured to said envelope extension and extendstoward said anode to constitute a continuation of said focus electrodeand an electron shield for shielding said shoulder portion from electronbombardment.
 8. An image converter in accordance with claim 3 in whichthe edge of said focus electrode closer to said anode structure isspaced from said anode structure in the direction of said electron-imagesource; and in which said envelope extension and said conductive ringare dimensioned so that at least a portion of said ring is spaced frombut is in overlapping relation with respect to the end portion of saidanode facing said electron-image source.